1. Field of the Invention
The present invention relates to a thin film circuit substrate for use in the microwave and millimeter wave regions, and a manufacturing method therefor. In particular, the present invention relates to a thin film circuit substrate using an organic insulating film as an insulating film that is located between an upper thin film electrode and a lower thin film electrode, and a manufacturing method therefor.
2. Description of the Related Art
In recent years, wireless communication application industries have been facing increased requirements of miniaturization, lower price, and higher performance for high-frequency devices used in high frequency regions such as the microwave region and the millimeter wave region.
Also, for the above-described high-frequency devices, transmission lines having low transmission loss and a high efficiency are required. While electrode materials having a low resistance are used for wiring lines (electrodes) to connect the transmission lines, it is believed that dielectric materials having a low dielectric constant and a low dielectric loss tangent are necessary for insulating films provided between the wiring lines.
In complying with such requirements, various wiring substrates have been developed, in which low-resistance materials such as Au, Cu, Ag, and Al are used as electrode materials, and organic resins having a low dielectric constant and a low dielectric loss tangent such as a polyimide resin, an epoxy resin, a benzocyclobutene resin, and a bismaleimide triazine resin are used as insulating films provided between the wiring lines.
One example of such a wiring substrate is a thin film circuit substrate as shown in FIG. 6, including a substrate 51 composed of a ceramic such as alumina, lower thin film electrodes 52 provided on the substrate 51, an organic insulating film 53 provided over the lower thin film electrodes 52, and upper thin film electrodes 55 that are provided on the organic insulating film 53 and are connected to the lower thin film electrodes 52 by via holes 54 provided in the organic insulating film 53.
In the thin film circuit substrate shown in FIG. 6, a problem arises in that the adhesion strength between the organic insulating film 53, which is an organic resin, and the upper thin film electrodes 55, which are made of an electrode material, is not sufficiently strong which causes delamination of the films in the process of forming wiring lines and in the wire bonding step of the wiring lines.
Furthermore, there is a problem in that the reliability of electroconductivity between the upper thin film electrodes 55 and the lower thin film electrodes 52 is decreased due to the fact that the surface of the lower thin film electrodes 52 is oxidized.
Accordingly, to improve the adhesion strength between the organic resin and an electrode material, various methods have been proposed, including:
(1) a method for improving the adhesion strength between the organic resin and an electrode material by surface-treating the surface of the organic resin with an oxygen plasma, as described in Japanese Unexamined Patent Application Publication No. 8-134639; and
(2) a method for improving the adhesion strength by providing a polar polymer on the organic resin, as described in Japanese Unexamined Patent Application Publication No. 9-219586.
However, in method (1) wherein the surface of the organic resin is treated with an oxygen plasma, although the adhesion strength between the electrode material and organic resin is improved, there is a problem in that the electrical properties of the organic resin, such as dielectric constant and dielectric loss tangent, are degraded due to oxidization of the surface of the organic insulating film, making it impossible to obtain the desired high-frequency module characteristics.
Furthermore, in method (2) wherein the adhesion strength between the organic resin and the electrode material is improved by providing a polar polymer on the organic resin, although the adhesion between the electrode material and organic resin is improved, there is a problem in that a step for polymerizing a polar monomer is needed after a step for activating the surface of the organic resin which increases the processing time, and results in increased production costs.
There are other possible methods for improving the reliability of electroconductivity between the upper thin film electrode and lower thin film electrode, such as removing the oxidized surface film on the lower thin film electrode by a wet etching method or a dry etching method including an RIE (Reactive Ion Etching) method.
However, when the oxidized surface film on the lower thin film electrode is removed by wet etching after the surface treatment of the organic insulating film, which is an organic resin, although it is possible to improve the reliability of electroconductivity between the upper thin film electrode and the lower thin film electrode, there is a problem in that the adhesion between the upper thin film electrode and the organic insulating film, which is an organic resin, is decreased due to the fact that the surface-treated layer of the organic insulating film in the case of (1) above and the polar polymer provided on the surface of the organic insulating film in the case of (2) above are combined with H2O.
Furthermore, when the oxidized surface film on the lower thin film electrode is removed by a dry etching method, such as an RIE method, after the surface treatment of the organic insulating film, which is an organic resin, there is a problem in that the surface-treated layer of the organic insulating film as well as the polar polymer provided on the surface of the organic insulating film are etched at the same time. Thus, it is not possible to improve the adhesion strength between the upper thin film electrode and the organic insulating film. In addition, the thickness of the organic insulating film is decreased because the organic insulating film itself is etched.
Furthermore, when the oxidized surface film on the lower thin film electrode is removed before the surface treatment of the organic insulating film, there is a problem in that during the oxygen plasma treating step and the polar polymer forming step in the process of surface-treating the organic insulating film after the removal of the oxidized film, an oxidized film is formed again on the surface of the lower thin film electrode. Thus, the reliability of electroconductivity between the upper thin film electrode and the lower thin film electrode is decreased.
Furthermore, in the above-described methods, the manufacturing process is complicated and production costs are substantially increased because the surface treatment step of the organic insulating film and the step for removing the oxidized surface film on the lower thin film electrode are two separate steps.
To overcome the above-described problems, preferred embodiments of the present invention provide a method for manufacturing a thin film circuit substrate, in which the surface treatment of an organic insulating film and the removal of an oxidized surface film on a lower thin film electrode are performed at the same time. Therefore, a thin film circuit substrate having excellent adhesion strength between the organic insulating film and the upper thin film electrode, as well as excellent reliability of electroconductivity between the upper thin film electrode and the lower thin film electrode is efficiently manufactured.
Additionally, a thin film circuit substrate having excellent adhesion strength between its organic insulating film and its upper thin film electrode as well as excellent reliability of electroconductivity between the upper thin film electrode and its lower thin film electrode is provided.
A preferred embodiment of the present invention provides a method for manufacturing a thin film circuit substrate including the steps of forming a lower thin film electrode or electrodes on a substrate, forming an organic insulating film over the surface of the substrate such that the insulating film covers the lower thin film electrodes, forming a via hole or via holes corresponding to a mask pattern in the organic insulating film using a patterning mask, irradiating the substrate from the side on which the organic insulating film is formed, with at least one type of ion of an inert gas selected from the group consisting of He, Ne, Ar, Kr, and Xe, such that the ion reaches the lower thin film electrodes through the via holes and removes the oxidized surface film on the lower thin film electrodes, and further generates, on the surface of the organic insulating film, at least one type of functional group selected from the group consisting of a sulfone group, a carboxyl group, a carbonyl group, and a phenol group, such that a modified surface layer with a surface modification coefficient of about 0.1 to about 0.5 is formed on the surface of the organic insulating film, the coefficient being determined by equation (1):
surface modification coefficient=total amount of the functional groups/total amount of C existing on the surface of the organic insulating filmxe2x80x83xe2x80x83(1),
and forming an upper thin film electrode or electrodes that are electrically connected to the lower thin film electrodes by the via holes, on the surface of the organic insulating film with the modified surface layer formed thereon.
According to the above-described preferred embodiment of the present invention, an organic insulating film is formed over a lower thin film electrode or electrodes formed on a substrate, a via hole or via holes are formed in the organic insulating film, and irradiation with at least one type of ion of an inert gas selected from the group consisting of He, Ne, Ar, Kr, and Xe is performed on the substrate to remove the oxidized surface film on the lower thin film electrodes, and to generate at least one type of functional group selected from the group consisting of a sulfone group, a carboxyl group, a carbonyl group, and a phenol group, on the surface of the organic insulating film, such that a modified surface layer with a surface modification coefficient of about 0.1 to about 0.5 is formed on the surface of the organic insulating film. Thus, the surface treatment of the organic insulating film and the removal of the oxidized surface film on the lower thin film electrodes are performed at the same time, and therefore, the production process for a thin film circuit substrate is greatly simplified. Furthermore, a thin film circuit substrate having excellent adhesion strength between its organic insulating film and its upper thin film electrodes as well as excellent reliability of electroconductivity between the upper thin film electrodes and its lower thin film electrodes is produced.
It is to be noted that the surface modification coefficient is defined by the ratio of the total amount of the functional group or groups to the total amount of C on the surface of the organic insulating film, and is determined by expression (1):
surface modification coefficient=total amount of the functional groups/total amount of C existing on the surface of the organic insulating filmxe2x80x83xe2x80x83(1).
The total amount of the functional groups and the total amount of C existing on the surface of the organic insulating film can be measured in atomic percentage, volume, or other suitable unit of measure, and is not particularly limited.
The following explains in more detail how to determine the surface modification coefficient according to preferred embodiments of the present invention with reference to FIG. 4.
First, the compositional ratio of C in each bond existing on the surface of the organic insulating film is identified by an X-ray photoelectron spectroscopy.
For example, FIG. 4 shows spectra of 1s of C obtained from the surface of an organic insulating film made of a benzocyclobutene resin by an X-ray photoelectron spectroscopy, in which the proportion of the area for each spectrum represents the compositional ratio (atm %) of C in each bond, and the total of the areas for the spectra represents the total atm % of C within its detection limit. It should be noted that a C1s spectrum represents the distribution of bound energy of the electron which orbits around a K shell among the electrons which orbit around an atomic nucleus of a C atom. Thus, the phrase 1s refers to 1 as the principal quantum number and s as the particular orbit of the electron which orbits around the K shell.
Accordingly, the compositional ratios (area ratios) of C in the Cxe2x80x94H bond and/or Cxe2x80x94C bond, in the xe2x80x94Cxe2x95x90O bond, and in the xe2x80x94COO bond shown in FIG. 4 are about 84 atm %, about 10 atm %, and about 6 atm %, respectively. Therefore, the compositional ratio of the functional groups existing on the surface of the organic insulating film to C existing on the same surface is about 16 atm %, that is, the sum of the approximately 10 atm % for the xe2x80x94Cxe2x95x90O bond and the approximately 6 atm % for the xe2x80x94COO bond. Therefore, the surface modification coefficient in this case is determined by the above-described equation (1) to be approximately 0.16 ({fraction (16/100)}=0.16).
It should be noted that the upper and lower thin film electrodes according to the present invention are not limited to so-called electrodes and include other components, such as transmission lines and pads.
There is no particular limitation to the shape of the substrate or the material for the composition, and substrates composed of various materials such as ceramics and having arbitrarily chosen shapes can be used in the present invention.
It is also to be noted that the reason that a range of about 0.1 to about 0.5 is chosen for the surface modification coefficient according to preferred embodiments of the present invention is that, when the coefficient is less than about 0.1, the adhesion strength between the upper thin film electrodes and the organic insulating film is insufficient, and when the coefficient is greater than about 0.5, deterioration of the electric properties of the organic insulating film occurs.
Another preferred embodiment of the present invention provides a method for manufacturing a thin film circuit substrate as described above, wherein the organic insulating film includes at least one resin selected from the group consisting of a polyimide resin, an epoxy resin, a benzocyclobutene resin, a bismaleimide triazine resin, an acrylic resin, and a cyclic olefin resin.
The organic insulating film including at least one resin selected from the group consisting of a polyimide resin, an epoxy resin, a benzocyclobutene resin, a bismaleimide triazine resin, an acrylic resin, and a cyclic olefin resin, generates at least one type of functional group selected from the group consisting of a sulfone group, a carboxyl group, a carbonyl group, and a phenol group on the surface of the organic insulating film. Therefore a modified surface layer having a surface modification coefficient of about 0.1 to about 0.5 on the surface of the organic insulating film is produced. Accordingly, the reliability of the organic insulating film is greatly improved.
Still another preferred embodiment of the present invention provides a method for manufacturing a thin film circuit substrate as described above, wherein each upper thin film electrode and lower thin film electrode includes at least one constituent selected from the group consisting of Cu, Ag, Al, Ni, Ti, Cr, NiCr, and Nb.
With each upper thin film electrode and lower thin film electrode including at least one constituent selected from the group consisting of Cu, Ag, Al, Ni, Ti, Cr, NiCr, and Nb, thin film circuit substrates having thin film electrodes with excellent electroconductivity are efficiently manufactured.
Still another preferred embodiment of the present invention provides a method for manufacturing a thin film circuit substrate as described above, wherein the etching rate of the organic insulating film by ion of the inert gas is not more than about xc2xc (one fourth) of that of the lower thin film electrodes.
By setting the etching rate of the organic insulating film by ion of the inert gas to be not more than about xc2xc of that of the lower thin film electrodes, adverse influences on the electric properties of the organic insulating film due to over-reduction of thickness of the film is effectively prevented, such that thin film circuit substrates having greatly increased reliability are manufactured.
Accordingly, by using an inert gas ion that provides a large difference of etching rates between the lower thin film electrodes and the organic insulating film, and that performs the surface treatment of the organic insulating film to generate functional groups, the surface treatment of the organic insulating film is efficiently performed and to remove of the oxidized surface film on the lower thin film electrodes is removed at the same time, resulting in a simplified manufacturing method and greatly reduced manufacturing costs.
Still another preferred embodiment of the present invention is a thin film circuit substrate including a substrate, a lower thin film electrode or electrodes provided on the surface of the substrate, an organic insulating film having a via hole or via holes, the organic insulating film provided to cover at least the lower thin film electrodes, and an upper thin film electrode or electrodes provided on the organic insulating film which are connected to the lower thin film electrodes through the via holes, wherein an oxidized surface film of the lower thin film electrode is removed from a region where the lower thin film electrode is connected to the upper thin film electrode through the via hole, and a modified surface layer is provided on the surface of the organic insulating film, the modified surface layer having a surface modification coefficient of about 0.1 to about 0.5 determined by equation (1):
surface modification coefficient=total amount of the functional groups/total amount of C existing on the surface of the organic insulating filmxe2x80x83xe2x80x83(1).
According to various preferred embodiments of the present invention, thin film circuit substrates that have excellent reliability of electroconductivity between the upper thin film electrodes and the lower thin film electrodes as well as excellent adhesion strength between the organic insulating film and the upper thin film electrodes are provided. The former advantage is achieved because the oxidized surface film on the regions of the lower thin film electrodes is removed, and the regions are connected to the upper thin film electrode through the via holes. The latter advantage is achieved because a modified surface layer having a surface modification coefficient of about 0.1 to about 0.5 is provided on the surface of the organic insulating film. Thus, the thin film circuit substrates according to preferred embodiments of the present invention are efficiently manufactured by the above-described methods for manufacturing a thin film circuit substrate.
Still another preferred embodiment of the present invention provides a thin film circuit substrate as described above, wherein the organic insulating film includes at least one resin selected from the group consisting of a polyimide resin, an epoxy resin, a benzocyclobutene resin, a bismaleimide triazine resin, an acrylic resin, and a cyclic olefin resin.
With the organic insulating film including at least one resin selected from the group consisting of a polyimide resin, an epoxy resin, a benzocyclobutene resin, a bismaleimide triazine resin, an acrylic resin, and a cyclic olefin resin, functional groups such as a sulfone group, a carboxyl group, a carbonyl group, and a phenol group are generated on the surface of the organic insulating film, such that a modified surface layer having a surface modification coefficient of about 0.1 to about 0.5 is provided on the surface of the organic insulating film, thus greatly increasing the reliability of the substrate.
Still another preferred embodiment of the present invention provides a thin film circuit substrate as described above, wherein the upper thin film electrodes and the lower thin film electrodes include at least one constituent selected from the group consisting of Cu, Ag, Al, Ni, Ti, Cr, NiCr, and Nb.
With the upper thin film electrodes and the lower thin film electrodes including at least one constituent selected from the group consisting of Cu, Ag, Al, Ni, Ti, Cr, NiCr, and Nb, the electroconductivity of the thin film electrodes is greatly improved, and therefore, the electrical properties of the substrate are greatly improved.
Further elements, characteristics, features and advantages of the present invention will become apparent from the following description of preferred embodiments with reference to the attached drawings.